Optical glass fibers are generally coated with two superposed radiation-cured coatings, which together form a primary coating. The coating which is in direct contact with the glass is called the inner primary coating and the overlaying coating is called the outer primary coating.
The inner primary coating is usually a relatively soft coating providing environmental protection to the glass fiber and resistance, inter alia, to the well-known phenomenon of microbending. Microbending in the coated fiber can lead to attenuation of the signal transmission capability of the coated fiber and is therefore undesirable. The outer primary coating, which is on the exposed surface of the coated fiber, is typically a relatively harder coating designed to provide a desired resistance to physical handling forces, such as those encountered when the fiber is cabled.
Such primary coating systems are typically prepared from radiation-curable, optical glass fiber coating compositions (hereinafter referred to as "radiation-curable composition"). It is a characteristic of such systems that the curing proceeds upon exposure to a radiation source, typically a UV-radiation source, for a time sufficient to provide a full cure of the coating compositions at the level of intensity of such source.
As the demand for coated optical glass fibers has increased, manufacturers must respond by adding more fiber drawing production lines and by attempting to increase the linear line speeds of the existing fiber drawing production lines. In the latter case, one factor which will determine the upper limit for the line speed will be the curing rate characteristics of the radiation-curable compositions, for a given radiation source and intensity.
If the line speed is increased to the extent that cure rate time requirements of the radiation-curable composition are not provided, the radiation-curable composition will not have received a sufficient amount of radiation to cause complete cure, or cross-linking, of the radiation-curable composition. The production linear line speed is generally inversely related to the amount of radiation striking the optical glass fiber. That is, as the production line speed is increased the amount of radiation exposure to the radiation-curable composition during the production process will necessarily decrease for a given radiation source. Incomplete cure of the radiation-curable composition is undesirable and must be avoided because then the desired protective properties of the incompletely cured primary coating may not be achieved and/or the incompletely cured primary coating may retain tackiness (giving problems in subsequent handling) or a malodorous odor may be present, and there may also be an increase in the extractables (undesirable) in the supposedly-cured coating.
Fiber production is therefore confronted with a problem that increases in production line speed are difficult to achieve without jeopardizing the cured coating quality.
If the production line speed is increased without careful consideration and balancing of the associated reduction in radiation exposure, then the radiation-curable composition may be processed at a radiation exposure level less than required for the desired level of curing, which means that the cured primary coating will not be fully cured. However, if the line speed is conservatively adjusted downwards to ensure that an adequate cure is achieved, this means that the line production is correspondingly reduced at the expense of product throughput.
Because the amount of radiation exposure is equal to the radiation intensity times the exposure time, the desired or required production line speed could be achieved by increasing the radiation intensity. This would require larger radiation units, which could lead to problems and costs in designing and operating the fiber coating line. Even though adjustments in the radiation intensity or exposure can be made, there remain certain fundamental practical issues associated with a radiation curing lamp assembly which can affect the actual amount of radiation reaching the radiation-curable composition.
Specifically, the amount of radiation striking the radiation-curable composition from, for instance, a UV-curing lamp system on a fiber drawing apparatus is not constant over the operative lifetime of the lamp and may be considered to be determined by the sum of the following:
(1) reflectivity of lamp reflector system, PA1 (2) intensity of curing lamp output, and PA1 (3) surrounding enclosure of radiation-curable material. PA1 (1) variability of reflector cleanliness, PA1 (1b) misalignment of reflector system with radiation-curable composition, PA1 (1c) solarization of the reflector system, PA1 (1d) the age of the lamp and system itself. PA1 at least one chromophoric indicator selected so as to be susceptible to destruction of its chromophoric characteristic upon exposure to actinic radiation and present in an amount which becomes substantially colorless when exposed to a level of radiation sufficient to cure said radiation-curable, optical glass fiber coating composition, wherein said at least one chromophoric indicator has a color which is distinguishable from a base color of said radiation-curable, optical glass fiber coating composition in cured form. PA1 providing a radiation-curable, optical glass fiber coating composition; PA1 measuring and determining a critical radiation dose level of said radiation-curable, optical glass fiber coating composition which is the minimum level of radiation sufficient to achieve a desired cure of said radiation-curable, optical glass fiber coating composition; PA1 selecting a chromophoric indicator having the characteristics of exhibiting a visible color while also having its chromophoric characteristics substantially destroyed upon exposure to actinic radiation, PA1 determining a concentration of a chromophoric indicator that exhibits a substantially colorless characteristic at said critical radiation dose level for curing of said composition; and PA1 incorporating at least said concentration of said chromophoric indicator into said uncured radiation-curable, optical glass fiber coating composition prior to application to a glass fiber. PA1 an optical glass fiber; and PA1 at least one radiation-cured coating containing a bleached chromophoric indicator, said at least one radiation-cured coating being formulated from a radiation-curable, optical glass fiber coating composition having as essential elements: PA1 drawing an optical glass fiber from a preform; PA1 applying a radiation-curable, optical glass fiber coating composition onto a bare optical glass fiber or onto a coated optical glass fiber; and PA1 exposing said coated optical glass fiber to radiation to cure said radiation-curable, optical glass fiber coating composition, PA1 using a radiation-curable, optical glass fiber coating composition which contains at least one chromophoric indicator in an amount which becomes substantially colorless when exposed to an amount of radiation suitable for curing said radiation-curable, optical glass fiber coating composition, wherein said at least one chromophoric indicator has a color which is distinguishable from a base color of said radiation-curable, optical glass fiber coating composition in cured form. PA1 (1) at least one coated optical glass fiber comprising: PA1 chromophoric indicator, said at least one radiation-cured coating being formulated from a radiation-curable, optical glass fiber coating composition having as essential elements: PA1 (2) a sheath covering said at least one optical glass fiber. PA1 at least one optical glass fiber coated with at least one radiation-cured coating containing a bleached chromophoric indicator, said at least one radiation-cured coating being formulated from a radiation-curable, optical glass fiber coating composition having as essential elements:
The reflector system's ability to reflect the radiation can vary during production runs due to:
The radiation curing UV lamp output typically changes in intensity as the bulb ages in use during production of the coated optical glass fiber increases. Moreover, the wavelength distribution of lamp emission can change as a result of its aging during such use.
When curing radiation-curable compositions on optical glass fibers, an elliptical reflector system containing a UV curing lamp is usually used. Such a system is shown in FIG. 1, (Prior Art).
As shown in FIG. 1, the UV lamp shown at 5 is positioned at one focal point of the elliptical reflector system shown at 3. A clear center tube shown at 7 is positioned around the other focal point of the reflector system shown at 3. The optical glass fiber shown at 9, having a liquid radiation-curable composition thereon, passes through the center tube 7. The clear center tube 7 is also flushed with an inert atmosphere such as nitrogen or argon gas to reduce the oxygen inhibition of polymerization. The clear center tube 7 also provides protection of the elliptical reflector system from contamination by the liquid coating as it is applied to the fiber, e.g. by splattering.
During the radiation curing production process, the inner surface of the clear center tube 7 may, over time, become contaminated with some of coating material or its components. This contamination has the effect of thereby decreasing the amount of radiation which reaches the uncured coating on the optical glass fiber 9 after passage through the center tube 7.
Thus, there is a need for some means to monitor and determine the level of cure of the glass fiber coating during the prolonged operation of the coating line. Since, once installed, the line apparatus and its radiation source are not readily changed except by expensive shut-down of the (generally continuously operating) line itself, the amount of radiation actually striking the radiation-curable composition present on the optical glass fiber will necessarily vary, depending on the condition at any given point in time as a result of the above-described problems presented by the reflector system, lamp output, contaminated center tubes.
It is accordingly difficult to confidently meet the demand for increased production line speeds while maintaining conditions which will assuredly provide optimum complete cure of the coating. At the present time, testing of the completeness of the coating cure is commonly done by off-line physical tests on specimens of the coated fiber after it has been produced.
What would be desirable is a system which would permit real time determination of the coating's cure level by some indicator means. Knowing whether or not the required complete cure is achieved under the operating conditions will then inform the line operator of the need to make adjustments to line speed, lamp intensity (if possible) or replacement, or equipment cleaning, while not jeopardizing wasted production due to an inadequate coating cure.
There has been no effective solution to the above described problems for the glass fiber coating technology, until the present invention.
U.S. Pat. No. 5.302,627 is directed to the fabrication of printed circuit boards and similar electrical or electronic devices. This patent discloses a method for indicating a cure point of an ultraviolet radiation curable composition used with such devices. It does not disclose any use of the compositions as coatings for optical glass fibers. Nor does it address the special problems of high speed continuously-operated glass fiber coating lines and the very critical requirements for such coatings. However, no definition of the cure point is provided. Furthermore, while this patent discloses that a dye can be used which becomes colorless upon exposure to ultraviolet radiation, no examples of such dyes are provided. All of the examples merely changed color upon exposure to ultraviolet radiation. This patent only teaches that the amount of dye used should be such that it does not inhibit the curing of the composition. One skilled in the art reading and comprehending this patent would not know how to prepare a radiation-curable composition which will provide instant real time visible feedback that the amount of radiation exposed to the radiation-curable composition is or is not sufficient to cure the radiation-curable composition to the desired level.
Published Japanese Patent Application No. 1-204902 is directed to molding materials, paints and adhesives. This patent discloses a photosetting resin composition containing a photocoloring compound which changes color upon exposure to light for finding the state of curing of the composition. It does not disclose any use of the compositions as coatings for optical glass fibers. Nor does it address the special problems of high speed continuously-operated glass fiber coating lines and the very critical requirements for such coatings. While this patent discloses that a dye can be used which becomes colorless upon exposure to ultraviolet radiation, no examples of such dyes are provided. All of the examples merely changed color upon exposure to ultraviolet radiation. This patent only teaches that the amount of dye used should be 0.5 to 10 parts by weight. One skilled in the art reading and comprehending this patent would not know how to prepare a radiation-curable composition which will provide instant real time visible feedback that the amount of radiation exposed to the radiation-curable composition is or is not sufficient to cure the radiation-curable composition to the desired level.